The present invention relates to speech packet switching method and device for telephone conversation by a packet switching system.
In recent years, a packet switching system in which an analog speed signal is converted to a digital signal by a pulse code modulation (PCM) system and the digital signal is packeted and transmitted via a packet switching network has been proposed and put into practice. In the speech packet switching system, speech and silence are discriminated and only the speech or a talkspurt is packeted as speech packet for transmission in order to reduce a burden to the packet switching network.
On the other hand, in the packet switching network, a transmission time of the packet through the packet switching network, that is, a network transmission delay time differs from packet to packet. This variance depends on a condition of the packet switching network or amount of traffic in the packet switching network. When the traffic in the packet switching network is in a steady state, the variance of the transmission delay time of the packet is constant but as the amount of traffic increases, the variance of the transmission delay time increases and as the amount of traffic decreases, the variance of the transmission delay time decreases.
The variance of the transmission delay time of the packet is an important factor in realizing the speech packet switching system. For example, in a receiving station, if a speech packet to be reproduced next has not yet been received due to the variance when a current speech packet was reproduced, a voice interruption occurs. Accordingly, in the speech packet switching system, a buffer is provided in the receiving station to absorb the variance and the speech packets are temporarily stored therein and appropriate delay times are intentionally added to the speech packets depending on the transmission delay times of the respective speech packets. More specifically, the speech packets having small transmission delay times in the packet switching network are retained in the variance absorbing buffer for a long time period to add long delay times while the speech packets having large transmission delay times in the packet switching network are provided with small additional delay times. In this case, a speech packet having a transmission delay time which is longer than a time period for which a speech packet having zero transmission delay time is retained in the variance absorbing buffer, that is, longer than a maximum additional delay time, is not reproduced but cut away because it is received after the reproduction time. In the speech packet switching system, the failure of arrival of the speech packet at the reproduction time is called a packet loss. As a packet loss factor increases, a speech quality is degraded. Experimentally, the packet loss factor of below 5% is permissible but when it is larger than 10% the degradation of the quality is not permissible.
In the prior art speech packet switching system, the variance absorbing buffer is fixed. Thus, if the transmission delay time of the packet increases due to the variance in time of the traffic in the speech packet switching network, the packet loss factor increases resulting in the degradation of the speech quality.